"Unfortunately, we were not able to process and thus print your product. Sadly we were not able to determine what exactly is wrong with it in an automated way.

We have already tried correcting the issue using our Mesh Medic. "

Now looking at it in Netfabb it appears to have 9 shells when it should only have one. Running the repair script appears to fix it and reduces it to 1 shell, but gives me no indication of where the extra shells where so I can go back to the original design and try any work out what is generating them.

In Blender, load the STL, in edit mode (press Tab) remove doubles (W key menu), select linked (aim to the big block and press L), hide selection (H key): you have 8 small tubes floating free and inside the big block, near the bolts (they are coaxial). Maybe you created them to be sure the bolts are solidly linked to the the big block?

Another way to see it.. Open the model in Netfabb. Select "Repair'
then right click the main shell (turning it green) and then hit Delete.

You'll see instantly where the 8 other shells are.

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In Netfabb, your other option is to select the main shell, and then right click it and select "Extract selected triangles as part".
Then use the Export option to save the new part, and just ignore the original shell(s).

Patience, Persistance, Politeness - the 3Ps will help us get us to Perfect Printed Products